Biochar amendment and Calamagrostis angustifolia planting affect sources and production pathways of N₂O in agricultural ditch systems

Nitrous oxide (N₂O) from agricultural ditches is a non-negligible source of anthropogenic greenhouse gas (GHG) emissions, but few studies have addressed this topic in depth. On the other hand, although there are numerous reports that biochar application can affect N₂O emissions from soil, the understanding of the process and source of changes is still incomplete. To examine the effect of biochar and Calamagrostis angustifolia on N₂O emissions, we conducted experiments with constructed ditches where corn stalk biochar (pyrolysis temperature of 450 °C) was applied at a rate of 16.77 Mg ha⁻¹ and C. angustifolia was planted. The sources (native sediment versus exogenous inorganic N) and production pathways (nitrification versus denitrification) of N₂O emissions were discriminated using the ¹⁵N isotope tracer method. We observed that biochar application reduced the cumulative total N-N₂O emissions from the native sediment by 10.8–18.7% and reduced the cumulative ¹⁵N-N₂O emissions from the exogenous ¹⁵N-labelled inorganic N by 25.7–68.6%; C. angustifolia planting reduced these cumulative N₂O emissions by 48.8–53.3% and 93.3–92.4%, respectively. The results showed that biochar stimulated nitrification and nitrification-derived ¹⁵N-N₂O emissions, but reduced denitrification-derived ¹⁵N-N₂O emissions in bare sediment microcosms; C. angustifolia effectively reduced both nitrification-derived and denitrification-derived ¹⁵N-N₂O emissions. Therefore, we concluded that the effect of biochar application on N₂O emissions may depend on its dominant N₂O production pathway and biochar application plus C. angustifolia planting could be beneficial for the mitigation of N₂O emissions in agricultural ditch systems.